Key one's left with? (6)
I believe the answer is:
island
'key' is the definition.
(thesaurus)
'one's left with?' is the wordplay.
'one's' becomes 'is' (roman numeral).
'left' becomes 'l' (common abbreviation).
'with?' becomes 'and' (I've seen this before).
'is'+'l'+'and'='ISLAND'
(Other definitions for island that I've seen before include "eg Rhode, Rhodes" , "Eg, eyot or inch" , "See 1" , "See 12" , "See 13" .)